Determination of adsorption isotherm of species adsorbed on roughened silver electrodes from in situ quantitative surface enhanced Raman spectroscopy

In situ surface enhanced Raman spectroscopy (SERS) has been used to determine quantitatively coverages of hemin and/or its reduced counterpart irreversibly adsorbed on roughened silver electrodes as a function of the applied potential in aqueous electrolytes. Analyses of spectral data recorded at pH 3 over a wide voltage region made it possible to obtain potential dependent adsorption isotherms, from which the standard redox potential was found to be −0.12 V versus saturated calomel electrode (SCE), in excellent agreement with data reported in the literature for hemin adsorbed on other substrates. Similar measurements performed at pH 7 not only revealed significant deviations from ideal Nernstian behavior, but the inflection point of the isotherm, i.e. −0.22 V versus SCE, was found to be more positive than that believed to be associated with the μ-oxo derivative of hemin, expected to be the majority species at this pH. Both these effects could be accounted for by invoking a fast pre-equilibrium between the electrochemically active (monomer) and electrochemically inactive (μ-oxo dimer) forms of hemin in the potential region in question. The value of E°=−0.21 versus SCE at pH 7 estimated from fits to the SERS data based on this model was in line with that for the monomeric rather than the dimeric form of hemin.